1. Field of the Invention
The present invention relates to a radio control apparatus and a link securing method for securing a communication link between a mobile station and a base station of a handover destination when the mobile station performs handover in a mobile communication system.
2. Related Background Art
In the mobile communication systems adopting the CDMA (Code Division Multiple Access) system, a mobile station simultaneously establishes communications with a base station of a handover source and with a base station of a handover destination on the occasion of handover, so as to perform diversity reception, thereby achieving improvement in communication quality and non-break communication during the handover (for example, refer to Japanese Patent Application Laid-Open No. 2001-251658). This handover technology is called diversity handover and, in general, a communication link is secured as below between the mobile station and the base station of the handover destination.
The mobile station receives pilot channels sent from the respective base stations of the handover source and the handover destination, and requests the diversity handover with inclusion of the base station of the handover destination when a difference between received powers from the two base stations becomes not more than a predetermined threshold.
In response thereto, the radio control apparatus in charge of the base station of the handover destination secures a communication link of the same information rate as the communication link used in communication between the mobile station and the base station of the handover source, for communication between the mobile station and the base station of the handover destination. This is because the information rates of the respective communication links need to be synchronized with each other in the diversity handover
However, if there occurs traffic congestion at the base station of the handover destination, the radio control apparatus might fail to secure the foregoing communication link in certain cases. In such cases, the mobile station repeats the request for diversity handover, and the problem of disconnection of communication will arise if no communication link is secured with the base station of the handover destination even after repetitive requests and if migration of the mobile station finally results in the cell of the base station of the handover destination becoming the first cell (a cell where the reception level of the pilot channel is maximum).